The Suiriri Flycatcher (Suiriri suiriri) of South America is represented by three distinct forms occurring in parapatry: (1) S. s. suiriri to the southwest in the Chaco/Pampas; (2) S. s. affinis located centrally in the Cerrado/southern Amazonia; and (3) S. s. bahiae to the northeast in the Caatinga. On the basis of an analysis of 366 specimens of S. suiriri, I found meager evidence for long-distance migration, little support for Bergmann's rule, and more support for Gloger's rule. I postulate that an ancestral population of S. suiriri split into three isolated populations, with the central population differentiating most rapidly into affinis, thus explaining the leapfrog pattern of greater similarity between peripheral suiriri and bahiae. After secondary contact, affinis freely hybridized with nominate suiriri in a hybrid zone to the southwest, where specimens demonstrate morphometric intermediacy and increased plumage variability; the rarity of parental phenotypes within the hybrid zone suggests that the two forms are conspecific according to the biological species concept. In the northeast, affinis may have hybridized with a remnant population of suiriri, possibly represented by poorly known bahiae. The intermediate size and increased plumage variability of bahiae resemble that of suiriri × affinis hybrids, supporting a hypothesis of hybrid origin for bahiae, but alternative hypotheses cannot be ruled out. This hypothesis of differentiation is supported by the concordant patterns of disjunction among several pairs of sister taxa of bird species that occur in the Chaco and Caatinga, with no intervening populations in the Cerrado, implying a shared historical process of vicariance. Genetic and behavioral studies are needed to elucidate further the status and history of differentiation within S. suiriri.